Patent Document 1 below discloses a substrate with a built-in electronic component that has a structure in which a core substrate made of an insulator has a plurality of penetrating holes penetrating the core substrate in the thickness direction, and electronic components are inserted in the respective penetrating holes. Electrodes (terminals) of each electronic component are connected to wiring lines on wiring boards that are disposed on the respective sides of the core substrate in the thickness direction.
In the actual substrate with a built-in electronic component that is configured in such a manner, the respective penetrating holes are scattered throughout the core substrate in a top view due to the wiring arrangement. As a result, the core substrate has a region of low rigidity (region where penetrating holes are formed in a concentrated manner, for example) and a region of high rigidity (region where no penetration hole is formed, for example). When the difference in rigidity between the two regions is not large, it is not likely to cause a problem such as described below. However, when the difference in rigidity between the two regions is large, a deformation of the substrate with a built-in electronic component such as warping or distortion may occur due to an external force such as vibration or pressure given to the substrate with a built-in electronic component, or an internal force such as thermal expansion/shrinkage occurring inside the substrate with a built-in electronic component. This problem is more likely to occur as the thickness of the core substrate becomes smaller, in other words, as the substrate with a built-in electronic component is made thinner.